Experiments were initiated to design and construct a respiratory apparatus for equine based on the indirect, open circuit method of calorimetry. The face mask, of light plexiglass, fit over the muzzle of the animal and was secured to the head, via a modified halter. Six one-way rubber inhalation valves were located along the lower portion of the mask permitting the animal to inspire atmospheric air. Four one-way rubber exhalation valves were situated in the false bottom of the mask. When the animal expired, the inhalation valves were forced shut and the exhalation valves were forced open, thus channeling the expired gas out of the mask. A series of flow meters, pumps, drying columns and storage containers were connected to the mask. Aliquots of expired gases were taken and analyzed for carbon dioxide and oxygen content to determine RQ and to calculate heat production values. All values were recorded at standard temperature, pressure and density. Three different diets were fed to equine in an attempt to alter the RQ: two ponies were first fed alfalfa-grass hay (Diet 1), then cracked corn and hay (Diet 2) was fed to 3 ponies and finally a commercial 18% protein grain mix and hay (Diet 3) was fed to the 3 ponies. Consumption problems occurred on some diets. Mean RQ's were .995 for Diet 1, 1.08 for Diet 2 and .956 for Diet 3. Mean heat production values were 2133.04, 2030.00 and 1727.23 kcal/kg feed/day for the alfalfa-grass hay, cracked corn and alfalfa grass-hay diet, and commercial grain mix and alfalfa-grass hay diet, respectively. / M.S.